(1) Field of the Invention
This invention relates to a high-purity transparent silica glass, and a process for producing the silica glass.
The high-purity transparent silica glass of the present invention is characterized, even when it is maintained at a high temperature for a long period, as exhibiting a good transparency and not developing color. Therefore, the silica glass is suitable, for example, for heat-resistant jigs for a semiconductor, flanges and furnace tubes.
(2) Description of the Related Art
In recent years, the degree of integration in semiconductor integrated circuits is more and more enhanced. Therefore glass not containing sodium element or potassium element as impurities giving a harmful influence on semiconductor integrated circuits is desired as a material for jigs used for the production of a semiconductor. Thus, for example, a member for heat-treating a semiconductor, containing not larger than 0.05 ppm of each of sodium, potassium and lithium elements, has been proposed in Japanese Unexamined Patent Publication (hereinafter abbreviated to "JP-A") No. S59-129421. It also is desired to reduce the content of iron element giving a harmful influence on a semiconductor. Thus, for example, silica glass having an iron content of smaller than 0.8 ppm has been proposed in JP-A H8-165134 and JP-A H8-175840.
In another aspect, the size of a wafer for semiconductor integrated circuits is becoming large year by year, and thus, the size of a silica glass jig for heat-treating a semiconductor also is becoming large. Therefore, a material for a silica glass jig for heat-treating a semiconductor, which exhibits a high viscosity at a high temperature, is required. In general a silica glass article exhibiting a high viscosity at a high temperature is made by melting a natural crystalline quartz powder.
A process for producing a silica glass ring has been proposed in Japanese Examined Patent Publication (hereinafter abbreviated to "JP-B") No. S35-791 wherein a ring-shaped mold cavity is filled with a crystalline silica powder and the silica powder is heated to 1,700.degree. C. or higher to be thereby molten. A process for producing a silica glass article has been proposed in JP-A H9-202631, JP-A H9-202632 and JP-A H9-183623 wherein a silica powder is heated to be thereby molten in a graphite mold characterized as having a silica glass layer interposed between the inner wall of the graphite mold and the silica powder filled therein.
In the case where a mold made of a heat-resistant metal or carbon is used for melt-molding a natural quartz powder, the mold is placed in vacuo so that the mold is not subject to oxidative degradation at a high temperature. Where a graphite mold made of general-purpose graphite is used, a problem arises such that the molten transparent silica glass becomes tinged with brown. Even if the as-molten transparent silica glass does not become tinged with brown, when the silica glass molding is fabricated, for example, by welding, and then annealed at a temperature of 900 to 1,400.degree. C. for about 20 hours to remove an internal stress, the resulting transparent glass article is tinged with brown, leading to lowering of the quality of glass article. However, the content of metal impurities in the transparent silica glass does not always have direct effect on whether the transparent silica glass becomes or does not become tinged with brown, and the reason for the brown color development further requires elucidation.
JP-B S35-791 has proposed the use of a mold made of molybdenum, which is a heat-resistant metal. This proposal has a problem such that the molybdenum is subject to oxidation at a high temperature to be thereby partially gasified, and the silica glass is contaminated with the gasified molybdenum.
When a mold made of graphite is used, the mold and the silica glass tend to be melt-adhered together by the reaction between the carbon and silica at a high temperature. Even when the melt-adhesion does not occur, the durability of the mold is becomes poor. Further, a graphite material generally contains metal impurities such as iron and calcium in a quantity of far larger than 1 ppm, which are gasified at a high temperature and the silica glass is contaminated therewith. This problem can be avoided by the above-mentioned process proposed in JP-A H9-202631, JP-A H9-202632 and JP-A H9-183623 wherein a graphite mold a silica glass layer interposed between the inner wall of the graphite mold and the silica powder filled therein. This proposed process is not advantageous from an economical view point because the silica glass is expensive and the fabrication of the silica glass into the layer to be fitted in the mold is troublesome.